Various processes are known for the production of higher linear alpha olefins (for example D. Vogt, Oligomerisation of ethylene to higher α-olefins in Applied Homogeneous Catalysis with Organometallic Compounds Ed. B. Cornils, W. A. Herrmann Vol. 1, Ch. 2.3.1.3, page 245, VCH 1996).
These commercial processes afford either a Poisson or Schulz-Flory oligomer product distribution. In order to obtain a Poisson distribution, no chain termination must take place during oligomerisation. However, in contrast, in a Schulz-Flory process, chain termination does occur and is independent of chain length. The Ni-catalysed ethylene oligomerisation step of the Shell Higher Olefins Process (SHOP) is a typical example of a Schulz-Flory process.
In a Schulz-Flory process, a wide range of oligomers are typically made in which the fraction of each olefin can be determined by calculation on the basis of the so-called K-factor. The K-factor, which is indicative of the relative proportions of the product olefins, is the molar ratio of [Cn+2]/[Cn] calculated from the slope of the graph of log [Cn mol %] versus n, where n is the number of carbon atoms in a particular product olefin. The K-factor is by definition the same for each n. By ligand variation and adjustment of reaction parameters, the K-factor can be adjusted to higher or lower values. In this way, the process can be operated to produce a product slate with an optimised economic benefit.
In WO-A-99/02472, there are disclosed novel iron-based ethylene oligomerisation catalysts that show high activity and high selectivity towards linear alpha olefins. The catalysts are based on iron complexes of a selected 2,6-pyridinedicarboxaldehyde bisimine or a selected 2,6-diacylpyridine bisimine.
In the present invention the term “bis-(aryliminoalkyl)pyridine”, or in short, “bis-aryliminepyridine” is used to describe both classes of ligands.
In a co-pending U.S. patent application Ser. No. 09/775,128 such systems are further improved, in particular with respect to the oligomer product distribution.
The bis-aryliminepyridine-FeCl2 based catalysts have been shown to be highly reactive towards ethylene but the reactivity towards other olefins such as propylene or higher alpha olefins has been found to be orders of magnitude lower.
B. L. Small and M. Brookhart disclosed in J. Am. Chem. Soc. 1998, 120, 7143-7144, that the oligomerisation of ethylene at a pressure of 400 psig (2.76 MPa) in the presence of a 50:50 volume ratio of 1-pentene to toluene as solvent and a bis-arylimine pyridine-FeCl2 based catalyst gave only ca. 3 mol. % of odd carbon number oligomers, thereby demonstrating the very high selectivity of such a catalyst for insertion of ethylene relative to alpha olefins.
Further experiments therein with a different bis-aryliminepyridine-FeCl2 catalyst showed even greater selectivity towards insertion of ethylene relative to the insertion of alpha olefins, with only traces (<1%) of odd oligomers produced.
The high selectivity of these catalysts towards ethylene was confirmed by the studies of V. C. Gibson et al., as disclosed in Chem. Eur. J. 2000, 6, 2221-2231.
Therefore, not surprisingly, the application of such catalyst systems has focused on products and processes with ethylene as feedstock and with preferentially no or little branching in products, for example, production of linear alpha olefins.